In general, centering is usually required for aligning a center of a workpiece 50 and a workpiece rotation axis of a machine tool, as shown in FIG. 6, especially to machine an outer circumferential surface after heat treatment during finish machining by grinding of the cylindrical workpiece or cutting of a hardened steel part. One example of such a machining apparatus of the prior technology is configured to perform grinding on an outer circumferential surface of the workpiece 50 with a grinding wheel 55 under conditions where tapered apertures of both ends of the workpiece 50 are supported by opposite centers 51. An attachment 52 is mounted on part of the outer circumferential surface of the workpiece 50. A kelly (lathe dog) 53 engages the attachment 52. Rotational driving power of a spindle 54 is transmitted to the workpiece 50 to rotate the workpiece 50 integrally with the spindle 54. See, Catalogue published by Kabuto MFG. Co., Ltd. (Page 8, Trade name “Kabuto Clipper”).
A machining device 57 is also known for machining a cylindrical workpiece 56 without using the kelly 53, as shown in FIG. 7. This machining device 57 adopts a machining method for grinding the outer circumferential surface of the workpiece 56 using a grinding wheel 59 while supporting the workpiece 56 with a centering apparatus 58. The centering apparatus 58 includes a pair of centers 60 and 61 oppositely arranged toward each other on an axis. One center 60 is detachably mounted on the tip end of a spindle 63 of a spindle unit 62. The other center pin 61 is also detachably mounted on the tip end of a spindle 65 of a tail stock unit 64.
According to this machining apparatus 57, the outer circumferential surface of the workpiece 56 can be ground by contacting the grinding wheel 59 against the outer circumferential surface of the workpiece 56 (see, JP 2003-245855 A).
However, it's problematic that the entire width of the workpiece 50 cannot be ground by one process. This is due to the fact that the kelly 53 prevents the lateral motion of the grinding wheel 55 when trying to grind of the workpiece 50 supported by the kelly 53 in a manner shown in FIG. 6.
On the other hand, when trying to grind the workpiece 56 supported by both the centers 60 and 61 as shown in FIG. 7, since the contact friction force between the centers 60 and 61 and the workpiece 56 is smaller than the machining force and since reduction of the machining speed is required, the machining period of time would be extended. Thus, the manufacturing cost would be increased. In addition, if one is trying to increase the pressing force of the centers 60 and 61 in order to make the contact friction force between the centers 60 and 61 and the workpiece 56 larger than the machining force, the large pressing force will sometimes deform the workpiece 56. This is especially true in thin-walled workpiece. Thus, the roundness of workpiece would be decreased. Accordingly, in order to ensure desired accuracy, it is necessary to perform grinding of the inner circumferential surface of the workpiece again after grinding of the outer circumferential surface. Thus, this increases the machining steps and accordingly the manufacturing cost.